Technical Reviews
On-farm assessment of soil quality in California’s Central Valley
Susan S. Andrews, Jeffrey P. Mitchell, Roberto Mancinelli, Douglas L. Karlen, Timothy K. Hartz, William R. Horwath, G. Stuart Pettygrove, Kate M. Scow, and Daniel S. Munk.
Agronomy Journal 94:12-23. 2002
This article summarizes results of side-by-side comparisons of alternative soil management practices that were part of the West Side On-Farm Demonstration Project, conducted from 1995 to 1998. It was funded through SAREP’s Biologically Integrated Farming Systems program. Data on practices such as cover cropping, and compost and manure applications were collected and used to test whether or not a soil quality index (SQI) could be applied to San Joaquin Valley soils. The study presents the first comprehensive soil property data set of this type for California farms.
Experimental Methods
Side-by-side comparisons were established at 11 farms in the fall of 1995 in the western San Joaquin Valley region between Huron and Mendota. Each site consisted of adjacent fields: one conventional and one alternative. Project fields were from 30 to 60 ha each. Consensus among participating farmers and researchers was that cover crop and compost or manure amendments would be used in the alternative field sites, but not in the conventional fields.
The soil amendment practices used in the alternative systems included various combinations of compost (either from chicken, steer or dairy manure, or cotton gin trash) and cover crops such as wheat, barley, and Sudangrass. Inputs that were incorporated into the rotations at each alternative farm site varied considerably in material and chemical composition, as did the crop rotation that was followed. Compost and manure applications ranged from 5.6 to 9 kg ha-1. No data are available on the general use of these amendment practices throughout the West Side region, however, according to the researchers, it is very unlikely that they are used on more than five percent of farmland annually. Interviews with project participants revealed that these soil amendments were being considered primarily as a means to add carbon to improve soil quality, rather than for fertility purposes or to reduce fertilizer inputs.
Six composite soil samples were taken from the alternative and conventional fields of each farm in the spring and fall of each year. Composite samples consisted of 8 to 12 subsamples taken from the surface 15 cm of soil. Mixed, air-dried samples were then taken to the University of California’s Division of Agriculture and Natural Resources Analytical Laboratory for the following chemical analyses: pH, electrical conductivity (EC), exchangeable potassium, organic matter, total carbon and nitrogen, cation exchange capacity, soluble zinc, manganese, and iron, soil aggregate stability, and microbial biomass carbon and nitrogen.
Soil quality index and statistical analyses
Researchers compared the alternative and conventional treatment means for six of the 11 farm sites. These six farms maintained treatment integrity and a commitment to the side-by-side comparisons over the course of the project. The expanded data set collected from a seventh farm in 1998 was used to construct the soil quality index (SQI). Techniques used to develop this index were similar to those previously applied in the Sustainable Agriculture Farming Systems (SAFS) project at the UC Davis campus. To construct the index, the researchers performed standardized principal component analysis of the data.
Results
Soil quality indicators that tended to change most frequently included total soil carbon, microbial biomass carbon and nitrogen, organic matter, exchangeable potassium and total Kjeldahl nitrogen (TKN). Soil organic matter (SOM) was significantly higher in the alternative/ amended fields at four of the six sites at the end of the project. Similar short-term changes in SOM in organic and low-input (cover cropped) cropping systems in the southern Sacramento Valley have been reported by other researchers. Microbial biomass carbon and nitrogen also were significantly higher in the alternative/amended fields at three sites in 1998. Biomass carbon and nitrogen were an average of 32 and 37 percent higher in the alternative fields relative to the conventionally managed fields, respectively. Properties not sensitive to the different soil management practices compared included potentially mineralizable nitrogen, pH and water stable aggregates.
Soil Quality Index
Based on principal component analysis of the expanded data collected from Farm 7, researchers identified the following characteristics as the minimum data set for the SQI: bulk density, zinc, water stable aggregates, pH, electrical conductivity, and soil organic matter. The SQI was calculated using a formula that incorporated weighting factors for each characteristic. Using this formula, the SOM and electrical conductivity variables appear to drive the soil quality index results. The organic system in Farm 7 had the highest index value. The SQI value for the transitional system was not significantly different from the manure, compost, or conventional systems. These results were similar to those from the UC Davis SAFS experiment where the organic and low-input plots consistently received higher SQI scores compared with the conventional treatments.
Data from this study indicate certain soil properties can be improved through simple practices that build soil organic matter. Researchers demonstrated that techniques used to compute soil quality indices in controlled experiments could be successfully applied to on-farm studies as well. These results emphasize that soil quality assessment is a useful tool for evaluating the effects of land management practices on soil function.
